Is this a woodpecker we're talking about? If so, try taping a strip of aluminum foil or cloth to your window. Something that will flap in the breeze and make him think twice about coming back (he will probably just move on to another window or other choice spot he likes, however). I've often wondered why they seem to think pecking on my house is more productive than pecking on a nice, juicy, insect-laden tree. 😒

This is a robin, and it happens every spring.I’ve tried hanging things in the window, but he’s pretty determined. We’ve had woodpeckers too, they sound like someone is jackhammering the house! We have a nature preserve behind us, you’d think they could find a nice tree among the hundreds that are back there.

Is he doing this just at this time of year? Maybe Your Mr. Peck is a male robin, sees his reflection in the glass, thinks it’s another male robin trespassing on his territory and trying to peck the ‘other bird’ away.
Another solution I’ve heard of is putting a picture of an owl or hawk in an ‘attack’ mode ie. Talons out, taped to the window, that will keep birds away.

We have a male woodpecker that loves our metal shed. That bill of theirs is like a jackhammer but at least it is not at our back window every year.

Bermuda, 3 years in a row we had a male robin that would throw himself against our picture window over and over and over trying to get at his 'rival'. The fact that, Wiggins, our big yellow cat sat in the window and watched him, and even jumpws at him occasionally, didn't deter him at all. The first year, we tried the owls, that didn't work at all either, he just moved on to other windows. We tried one of those big plastic hawks, (we managed to make it look like it was sitting on the branch), that worked for about a week, and then he got wise to the fact that it never moved. We finally resorted to covering our wonderful sunny windows with non-reflective plastic so that he couldn't see his reflection anymore. Then we spent the rest of the summer watching him throw himself at our neighbors garage windows, their truck windows, our truck windows, anything he could see his reflection in. It was pretty nerve racking, I kept expecting to find him dead on the ground somewhere around the house every day from the beating he gave himself. For the next two years, as soon as we heard that thud, we'd cover the windows until he finally found a mate and left off. He didn't come around this spring, there was a new robin singing in our maple. Hopefully, he's at peace somewhere - one way or another.

He left for a bit yesterday, but was back at 6:30 this morning! The bricks under the window are covered with white polka dots...

See photo.....this is what keeps birds from our glass doors. They think it’s a predator.